The invention relates to a method and an apparatus for controlling transmitting power of a communication system, and more particularly, to an automatic level controller for a WiMAX system and a related method thereof.
In a communication system, the signal to noise ratio (SNR) is an important indicator of the transmission quality of the communication system. General method for improving the SNR is to increase the power of transmitted signals. This method, however, does not normally work well in wireless communications, especially under a condition where many users utilize adjacent frequency bands in a small area (e.g. in a communication cell) at the same time. This is because a first signal of large power might become a significant interference source for a second signal adjacent to the first signal in a frequency band. Although the SNR for the first signal is improved, the SNR for the second signal is degraded. Hence, wireless communication systems, e.g. WiMAX systems and CDMA systems, etc., usually define a maximum allowed transmitting power for each user. In this way, signals in a wireless communication system will not interfere with each other.
For a transmitter, a target transmitting power of a signal is well calculated, and is carried out by the coordination of elements of the transmitter wherein the element having the biggest influence is (for example) a power amplifier (PA) of the transmitter. The PA amplifies power of a radio-frequency (RF) signal to be transmitted and feeds the amplified RF signal to an antenna of the transmitter. The characteristics of the PA, however, vary with many non-ideal effects, e.g. component aging and surrounding temperature variation, etc. This results in a deviated actual transmitting power that is different from the target transmitting power. The deviation can result in the actual transmitting power surpassing the target transmitting power, and furthermore, surpassing the maximum allowed transmitting power of the wireless communication system. A significant interference for other users will therefore occur.